Which concept was that, exactly?

I’m normally reluctant to disagree with Brad DeLong, because he usually writes about stuff that he understands and I don’t. But I find myself puzzled by his post about capital punishment.

It seems that the death-penalty debate has been the subject of dueling attempts to turn professors’ prejudices into ersatz expertise (anti here and pro here). [Orin Kerr at the Volokh Conspiracy wonders why anyone should care about what political opinions a bunch of professors hold, as opposed to caring about what their research finds.]

One of the anti-capital-punishment points is the risk of wrongful execution. Some of the pro-death-penalty folks claim that the risks of that are exaggerated by those who assume that every overturned conviction represents a case of actual innocence, and assert that of the 100 cases of wrongful capital sentencing in over the past 30 years, only 30 involved demonstrable innocence.

DeLong headlines his post “Unclear on the Concept Department” and comments:

Here we have a bunch of people who seem unclear on what they are arguing for. For them to say that the system they favor was not on its way to frying 100 innocent people, but only on its way to frying 30 innocent people is not a powerful argument in their favor.

Well, compared to what? If we think of capital punishment as part of an attempt to reduce the incidence of homicide, then we might want to use the size of the problem as a benchmark. In the 30 years since executions were resumed, there have been roughly 500,000 murders and non-negligent manslaughters in the United States. There have been somewhere between 5,000 and 10,000 capital sentences handed down, and something under 800 actual executions.

Against that background, 30 wrongful sentences — not wrongful executions, let’s note — doesn’t seem like a very large number. Is one too many? Sure. But so is one homicide. If capital punishment were able to reduce the incidence of homicide by a tenth of one percent, that would be about 18 avoided killings per year. How does that stack up against one wrongful death sentence per year?

Or perhaps we should compare the injustice of wrongful capital sentencing against the injustice of wrongful imprisonment. Thirty bad sentences out of (let’s say) 6,000 would be a rate of one in 200, or half a percent. The rate for non-capital offenses is unknown, but almost certainly higher: I’d guess that about 2% of the 400,000 people who go to prison each year are actually innocent of the crime charged. That’s 8,000 wrongful imprisonments per year. And unlike someone on death row, an ordinary prisoner has only a negligible chance of having his conviction reversed.

So one wrongful capital sentence per year would be a very small number compared either to the homicide problem or to the miscarriage-of-justice problem, and obviously it would be a smaller number than three per year. That leaves me unclear on what concept the pro-death-penalty professors are accused of being unclear about.

[As faithful readers will know, I really have no dog in the death-penalty fight. I’m queasily for it in principle, but in practice I assume that any politician who makes an issue of being for it is a scoundrel and unserious about the real issues of crime control. Further thoughts on the death penalty here and here.]